Conventional mirrors receive incident light and re-emit reflected light. Conventional mirrors include a transmissive glass substrate and a reflective coating covering the side of the mirror opposite incident light. The glass substrate is typically a thin sheet of uniform thickness. The reflective coating is typically a metallized or dielectric layer of relative uniform thickness and coverage over the substrate surface. Conventional mirrors reflect light using interactions between the magnetic portion of an electromagnetic field associated with incident light and oscillating electrons resident in the reflecting layer dielectric or metal layer. Incident light comprises photons. Operatively, incident photons interact with oscillating, free electrons, disposed within the reflective layer. The incident photons interact (e.g. collide) with free electrons in the reflective layer, thereby increasing their energy level, and causing the electrons to re-emit the energy in the form of a re-emitted photon as the electron drops back to a lower energy level. At a macro level, these re-emitted photons constitute the reflected wave front returned by the mirror when illuminated with incident light.
Conventional mirrors necessarily include a physical edge about their periphery. Because the region beyond the edge contains no oscillating electron to excite, the mirror edges constitute electric field discontinuities. The electric field edge discontinuities induce a corresponding change in the electric field of the mirror about the glass substrate adjacent to the mirror periphery, the electric field change being greatest at the mirror edge and becoming smaller from the mirror edge toward the mirror's center. This field discontinuity causes a diffraction effect in conventional mirrors whereby a portion of light incident to the glass substrate at the mirror periphery is diffracted due to the resultant impedance mismatch between the mirror and the free space beyond mirror edge. Conventional mirrors diffract about 20% of incident light outside of the core reflected image. This diffraction effect results in an effect known as an ‘Airy rings’ visible in the reflected image intensity profile.
FIG. 1A shows a top down, plan view of an exemplary Airy disk 10. Airy disk 10 is diffraction pattern resultant from light reflected (re-emitted) from a conventional mirror (not shown), and shows a central bright region 14 and a plurality of concentric bright rings 12 arranged concentrically about center bright region 14. FIG. 1B similarly shows a three-dimensional perspective view of Airy disk 10, shows the central bright region 14 and the plurality of concentric bright rings 12 arrayed about the central bright region 14.
Airy disks represent information loss between light incident to the mirror and the mirror reflection corresponding relative intensity of concentric bright rings to the central bright region. Since a common purpose of incorporating a conventional mirror into an optical system is to redirect light conveying information without information loss, minimizing the Airy disk associated with the mirror is desirable. However, it is thought to be impossible to avoid this effect in ordinary mirrors, and optical system designers therefore typically seek to minimize the use of mirrors when designing optical systems—particularly in systems that are extremely sensitive to information loss such as space-based observation systems and lithographic workstations for semiconductor manufacturing. This is because, at the edges of conventional mirrors, the electric field discontinuities that produce diffraction of light due to an impedance mismatch between the mirror and the free space beyond the edge of the mirror.
One approach to the edge diffraction effect of conventional mirrors is physical apodization. Physical apodization limits diffraction by limiting the aperture in the system, either at the primary mirror or at an aperture stop in the system. This limits surface area of the mirror receiving incident light, and approach reduces the effect of the edge impedance mismatch by limiting incident light to regions of the having a smaller amount of mismatch. Such diffraction suppression techniques results in moving the diffraction pattern features around or effectively decreasing the collecting area such that no increase in signal to noise was possible. Physical apodization also requires the use of a tapered edge that effectively directs light away from the optical system. This has not proven useful in the visible spectrum, and has primarily been used in the microwave portion of the spectrum.
Other approaches to edge diffraction in conventional mirrors are apodized intensity and patterned apodization. Apodized intensity is a technique whereby the intensity of the reflection is decreased near the edge of the mirror. Patterned apodization is technique whereby a petaled edge imposes intensity apodization through the geometry of structures constructed about the periphery of the mirror surface. Both these alternative approaches, however, do not yield a net increase in the signal to noise of the reflected light relative to the incident light, and result in nulls in specific image locations.
Consequently, there exists a need for a mirror that addresses the edge diffraction problem associated with conventional mirrors. There exists a further need for a mirror which yields an effective improvement of signal to noise in the reflected light relative to the incident light. Such a mirror should be able to be fabricated using conventional manufacturing processes and on a mature equipment set, and be suitable for enhancing imaging systems and for optical systems that convey information using light.